1. Field of the Invention
This invention relates to apparatus for the holding and refining of molten aluminum. More particularly, it relates to the protection of heating elements used in such apparatus.
2. Description of the Prior Art
In the refining of aluminum, the use of externally heated, refractory lined cast iron tubs as the refining vessels has been found to be disadvantaged by the limited and somewhat unpredictable life of said tubs. This undesired condition results from the failure of the cast iron tubs because of cracking, bulging, chloride corrosion or wash-out. In addition, design constraints pertaining to such cast iron tubs result in the use of configurations that are difficult to clean, creating a further practical disadvantage to their use in commercial operations.
In an effort to overcome such disadvantages, a refining system was devised consisting of a refractory lined vessel having vertical tubular immersion heaters, such as silicon carbide tubes with internal helical resistance heating elements, suspended from the refining vessel cover. In this case, the heaters were found to have a limited life and, in practice, were very hard to replace. When a heater failed because of the breakage of the silicon carbide tube, pieces of the broken tube would frequently cause breakage of the spinning nozzle used to inject gas into the molten aluminum within the vessel. In addition, such a system was very difficult to clean because of the many recesses between adjacent heater tubes, and between the heater tubes and the vessel walls, where dross would accumulate and be difficult to remove in a convenient manner.
As a result of such problems, an improved apparatus was developed for the refining of aluminum or other molten metals. This apparatus comprised an all-refractory system in which two opposite side walls each consisted of a graphite block having electrical heating elements positioned in vertical holes provided in the graphite blocks, said holes being open at the top and closed at the bottom thereof. Various other features of this system are disclosed in the Szekely patent, U.S. Pat. No. 4,040,610. This system thus provided an internal heating source while overcoming the drawbacks associated with the use of the immersion heater. It was found to increase heater life, minimize erosion and facilitate repair of the system. For the holding of aluminum in the molten state, such a system comprised a vessel adapted for the holding of aluminum in a molten state and including an insulated shell impervious to molten metal, a lining comprising graphite blocks for a portion of the interior of the shell that is intended to be below the surface of melt, and at least one heating means disposed within one or more of the blocks. For application in the refining of aluminum, the system also included at least one rotating gas distributing means disposed in the vessel, as well as inlet and outlet means for molten metal and for gases.
Refractory systems using such graphite heater blocks have been found to constitute a desirable improvement in the art and have been employed to advantage in commercial aluminum refining operations. Nevertheless, further improvements in such systems are desirable to enhance their suitability by the overcoming of practical operating problems encountered in commercial operations. The main problem thus encountered relates to the relatively short life of the heaters under commonly employed operating conditions. One mode of such failure results from oxidation of the graphite heater block, usually from the top portion thereof. Upon oxidation of the block from the top portion and then downward to below the operating metal level in the refining or holding vessel, molten aluminum is able to flow through the oxidized block into the heater cavity, thereby shorting out the electrical heating element positioned therein. Furthermore, where chlorine is employed in the process gas employed, liquid chlorides formed in the molten aluminum may pass through the graphite block and accumulate in the bottom of the heater cavity, leading to the shorting out of said electrical heating element positioned therein. In addition, corrosion of the heaters and heater connections is found to occur as a result of the passage of liquid or vapor state chlorides, that are generated during the refining of aluminum, from the refining chamber, through the interconnected porosity of the graphite block, and into the heater zone in said graphite block. The overcoming of such causes of the relatively short life of the heaters constitutes a significant advance in the development of aluminum holding and refining vessels.
One suitable approach for overcoming such short heater life is disclosed in the Pelton patent, U.S. Pat. No. 4,717,126. The refractory system heater arrangement shown therein is a combination of elements that are designed to (1) protect the graphite heater block from oxidation, and (2) protect the heating element positioned in the block from the attack of chlorides. In the zone of the heater, this dual function is performed by the use of metal heating element container 12, referred to herein as a heater can. In the approach of the Pelton patent, this heater can is sealed to support plate 9. The heating element positioned in the heater can is exposed to air, which is free to enter the interior of the heater can. In this arrangement, the heater can acts as a double barrier to (1) prevent air from the heating zone within the heater can from reacting and oxidizing the surrounding graphite block, and (2) prevent chlorine and chlorides that penetrate the graphite block from contacting and corroding the heating element therein.
While the heater arrangement disclosed in the Pelton patent can be used successfully to prevent oxidation of the graphite block and the attack of chlorides on the heating element positioned therein, there is, of course, a continuing need and desire in the art for further improvement in the aluminum refining art. More specifically, there is such a need and desire for simplification, reduced costs and improved performance of aluminum holding and refining systems. With respect to all-refractory systems and to the protection of heating elements therein, the heater cans represent an expensive additional element of the system. In addition, the use of such heater cans, however useful for the intended purpose, tends to limit the heating power that can be usefully installed in the graphite block.
It is an object of the invention, therefore, to provide an improved electrically heated apparatus for the holding and refining of aluminum.
It is another object of the invention to provide a simplified arrangement for the positioning of heating elements within graphite heater blocks of aluminum holding and refining vessels.
It is a further object of the invention to provide an aluminum holding and refining vessel in which the graphite block in which the electrical heating element is positioned is protected from oxidation and said heating element is protected from chloride attack without the need for employing metal heating element cans.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.